Atomic Absorption and Atomic Fluorescence Spectrometry Section A

2. Atomic Absorption and Atomic Fluorescence Spectrometry Section A By Matt Boyd, James Joseph, Jon Blizzard, Jackie Freebery, Hunter Bodle

3. Atomization Techniques • AAS and AFS • Two techniques • Flame Atomization • Electrothermal Atomization

4. Flame Atomization • Analyte is nebulized by flow of gaseous oxidants • Desolvations • Dissociation • Volitalized

5. Types Flames • Figure 9-1

6. Flame Structure • Primary Combustion zone • Blue region, rarely used for spectroscopy • Interzonal region • Most widely used part • Secondary combustion zone • Products of inner core disperse

7. Flame Atomizers • Uses • Atomic Absorption • Fluorescence • Emission Spectroscopy • Laminar-flow Burners are Commonly Used • Aerosol, oxidant, and fuel are burned in flame • Performance Characteristics • Most reproducible

8. Electrothermal Atomizers • Provides enhanced sensitivity • Operates evaporating sample at low temps • Ashing at higher temp • Measures absorption and fluorescence • Used in the ICP

9. Electrothermal Atomization • Occurs in open ended cylindrical graphite tube • Held between two contacts in water cooled housing • Two inert gas streams are provided

10. Output Signal • Transducer output rises to a maximum • Rapid decay back to zero • Quantitative determinations • Peak height • Peak area

11. Performance Characteristics • Advantages • Sensitivity • Relative precision • Disadvantages • Furnace methods • Analytical range

12. Analysis of Solids with Electrothermal Atomizers • 1st- weigh grounded sample into a graphite boat and insert boat into furnace. • 2nd- prepare slurry of powdered sample by ultrasonic agitation in an aqueous solution. The slurry is then pipetted into furnace atomization.

13. Specialized Atomization Techniques • Glow-Discharge Atomization • Hydride Atomization • Cold-Vapor Atomization

14. Chapter Nine:Atomic Absorption and Atomic Fluorescence Spectrometry Section 9A: Sample Atomization Technique By Rachel Conroy Katie Payne

15. Flame Atomization • Sample is nebulized by a flow of gaseous oxidant and fuel that carries it to a flame • Process • Desolvation • Volatilization • Dissociation • Ionization • Excitation to form spectra

16. At each phase of atomization spectra can be obtained.

17. Types of Flames • Oxidants • Air: 1700oC to 2400oC • Oxygen • Nitrous oxide • Burning velocity states when flame is stable • Too low: causes flashback • Too high: flame will blow off

18. Table 9-1 Properties of Flames

19. Flame Structure • Primary Combustion Zone • Interzonal Area • Secondary Reaction Zone • Flame Profile

20. Flame Atomizers Variables • Fuel and Oxidant Regulators • Double-diaphragm pressure regulators • Rotameter • Performance • Most reproducible • Low sensitivity

21. Schematic of a laminar-flow burner, the typical atomizer used in AAS.

22. Electrothermal Atomization • Long residence time • Measurements and vaporization • Evaporated at a low temperature • Ashed at a higher temperature

23. Electrothermal Atomizers • Graphite tube • 2 inert gas streams provided • Transverse configuration • Pyrolytic carbon seal

24. Shown is the cross-sectional view of a graphite furnace atomizer. The L’vov platform and its position in the graphite furnace.

25. Other info • Output Signals • Measures peak height • Performance • Slow because of cooling cycles • Analytical range is narrow • High sensitivity • Analysis of Solids • Finely ground samples, slurry

26. Specialized Atomization Techniques • Glow-discharge atomization • Hydride atomization • Cold-Vapor atomization

27. Atomic Absorption Instrumentation9-B Brian May Mandi Kauffman Tyler MacPherson Carolyn Inga Ginny Harrison

28. Atomic Absorption Instrumentation • The AAS Consists of… • A radiation source • Sample Holder • Wavelength Selector • Detector • Signal Processor • Read Out

29. Radiation Sources • Potentially highly specific because of narrow absorption lines. • These narrow lines also cause problems because a linear relationship between absorption and concentration requires narrow source bandwidth relative to the width of an absorption line, but even good monochromators have bandwidths significantly larger than the absorption lines.

30. Problems Created • Non-linear calibration curves are inevitable when the AA is equipped with an ordinary spectrophotometer and continuum radiation source. • Small calibration curves are obtained because only a small amount of the radiation from the monochromator slit is absorbed by the sample, this gives poor sensitivity

31. Solutions • The use of bandwidths narrower than the absorption lines. This is done by exciting the atoms with a lamp, filtering the light, and choosing appropriate operating conditions(source temperature and pressure). • This disadvantage to this method is that it require an additional source lamps for each element, or group of elements.

32. Hollow Cathode Lamps (9B-1)Sample

33. -Most common source for atomic absorption measurements • -Consists of a tungsten anode and a cylindrical cathode sealed in a glass tube which is filled with either argon or neon gas a pressure of 1-5torr • -Cathode is constructed from the metal whose spectrum is desired (or, if not constructed from the metal, it then serves to support a layer of that metal)

34. -When a potential of about 300V is applied across the electrodes, ionization occurs of the inert gas (argon or neon). The current is generated (of about 5-15 mA) as ions and electrons migrate to the electrodes. • -if potential is large enough the gaseous cations gather enough kinetic energy to dislodge the metal atoms from the cathode surface and produce an atomic cloud in the process known as sputtering. • -The excited metal atoms (a portion of those sputtered) emit their characteristic radiation as they return to ground state

35. -Efficiency of the cathode depends on its geometry and the operating potential: • High potentials (and thus high currents)  greater intensities • -A down-fall to high currents is that they produce an increased number of unexcited atoms in the cloud which have the potential of absorbing the radiation emitted from the excited atoms (Self-absorption) • -This leads to lower intensities

36. Electrodeless Discharge Lamps

37. What are they made of? • Sealed quartz tube • Filled with an inert gas (Ar) • Small amount of metal or its salt

38. What does it use? • Uses radio frequency • Or microwave radiation to energize it

39. What happens? • The gas is ionized by the frequency • Once enough energy is obtained it excites the atoms of metal • The metal spectrum is the desired spectrum.

40. What it provides • Provides radiant intensities in greater supply than a Hollow-Cathode Lamp (HCL) • Not as reliable as the (HCL) • But better for elements such as • Se, As, Cd, Sb

41. Source modulation • Emitted radiation is removed via the monochromator • It is necessary to adjust the output the source so intensity will fluctuate at a constant frequency

42. Detector receives 2 signal • An alternating from the source • Continuous from the flame. These signals are then converted into electrical responses A high pass RC filter (section 2B-5) can be used to remove unadjusted signals

43. Adjusting the emission can be done by inserting a circular metal disc (chopper) into the system between the source and the flame • Rotation of this disk at a constant rate will create a beam that is “chopped” to the desired frequency • Tuning forks with vanes attatched to alternately allow the beam to pass and to not pass is another technique • An alternative is the power supply being designed for intermittent or ac operation so the source can be switched oin and off at the desired frequency

44. AA SpectrophotometerSee Figure 9-13 for block diagrams • Instrument must be capable of providing a sufficiently narrow bandwidth to isolate the line chosen for the measurement • Glass filter – alkali metals • Only a few widely spaced resonance lines in the visible region • Separate filter and light source for each element • Most use photomultiplier tubes

45. Single-Beam • Several hollow- cathode sources • Chopper or pulsed power supply • Atomizer • Simple grating spectrometer with a photomultiplier transducer • 100% transmittance is set with a blank • The blank is replaced with samples to determine absorbance and transmittance

46. Double Beam • Beam from hollow-cathode source is split by a mirrored chopper • One half passes through the flame and the other half goes around it • 2 beams recombine by a half silvered mirror and passed into a Czerny-Turner grating monochromator • Photomultiplier = transducer • Output = input to a lock-in amplifier • Ratio between reference and sample is amplified and fed to the readout • Since reference beam is not passed through the flame it cannot correct for loss of radiant power due to absorption or scattering by the flame

47. Chapter 9 Section C Megan Seeger, Andrea Lando, Joe Bailey, and Sarah Duncan

48. Spectral Interferences • Can be caused by overlapping lines but is very rare due to the emission lines of the hollow-cathode sources being so narrow • Can also result from the presence of combustion products that exhibit broadband absorption or particle products that scatter radiation • Both reduce the power of the transmitted beam and lead to positive analytical errors

49. Continued • A more troublesome problem occurs when the source of absorption or scattering originates in the sample matrix • Interferences because of scattering by products of atomization is most often encountered when concentrated solutions containing elements such as Ti, Zr, and W are aspirated in the flame

50. Continued • Interferences caused by scattering may also be a problem when the sample contains organic species or if organic solvents are used to dissolve the sample • Flame atomization spectral interferences by matrix products are not widely seen and can be avoided by variations in the analytical variables